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ok 92% 93 94 99.... percent is not what I'm gettin at here so please don't go there.... What I am thinking here, I want another 400 opinions but here goes: Seems to me with tubular heat exchangers, The clamshell heat exchanger has a LOT more surface area, so when a furnace calls for heat and starts blowing, it would seem to me that a furnace would have to run a little longer trying to get heat off the tubular into the house than with a clamshell.... hmmmm In other words a tubular would cool down a little faster, making the furnace have to run a little longer to get the same heat??? I haven't noticed a difference but how do ya know...... I guess what I'm gettin at here is 92% is fine but thats not going to cover a longer run time.... total fuel deal..... Anyway without a lot of insults....what do you guys make of this thinkin?....

Tubular heat exchangers do cool down quicker than clamshells, yes. But they also heat up faster and generally a tubular heat exchanger is hot as mentioned all the way. With a tubular HX you can force air all around it with a clam shell, your limited to running air up the two sides only. There is no doubt that in the dead of winter the older furncases that were made of cast iron, held thier heat quite well and became more efficient then when they cycled infrequently. I suppose there are pros and cons to both. Personally I prefer a tubular.

I think you'll be surprized at the amount of surface area on tubular heat exchangers.

100,000 Btu per hour is the same no matter what the configuration of the heat exchanger, so there is no longer run time with a tubular of any other style heat exchanger in a furnace of the same capacity rating.

Like doc stated, tubular heat exchangers cool down faster and heat up faster. In other words, they transfer heat faster. That is what it is all about; exchanging heat/heat exchanger.

How fast a unit heats up and cools down is a factor in the AFUE calculations. As Robo says infers, the input is the input and the AFUE rating compnsates for heat up and cool down speeds in the calculation & tests.

how long have tubes been around now.,...15-20 years?where are these most likely to crack/bust at?...the inspector camera i seen doesn't look like it will reach all the way in thru the tubes.With heating up and cooling down quicker wwouldn't these need to expand a little?Are the welds on these the weak spot?.....just a couple questions i've been wanting to ask..........

Wrong forum to discuss I think.
From my perspective, there is no likely place to crack as the designs are rigourously tested AS STATED IN THE INSTRUCTIONS under statics and ducts as lined out in the instructions.
Problems arise when installers run the units at too high of statics, with only one side cut out (return) when two are called for, blocking off one orifice to de rate a unit, over/under firing a unit because no one checks rate to see what the units are firing at etc.
I called various dealers in two states and asked what their heating value was. The responces blew me away as most didn't know what I was talking about. The common installer answer resembled " I adjust the blower speed to get it in the rise range; I dont normally check it isn't that the manufactureres responsibility? The gas companies responces were worse "You need to talk to our marketing representatives" like it was top secret. Then, when I would get an answer it would be the altitude de-rated BTU instead of the sea level standardized (which is what I asked for).I heard heating values from 750 to 1050 BTU ft^3. Thats roughly a 30% swing. Little help here!
So, my answer is there are too many things to consider. Tubes will fail on an over fired condition, Transitions will fail on bad airflow conditions, water will rust them out on under fired conditions.
And you want to know what will fail or is the most likely to have the highest stress? under what conditions? Thermal stresses are compensated for in the design. Manufaturers install orifices to a 1050 heating value if their AGA/CSA certified units. If you are at altitude the issues are multiplied.

Originally posted by Modman So, my answer is there are too many things to consider. Tubes will fail on an over fired condition, Transitions will fail on bad airflow conditions, water will rust them out on under fired conditions.
And you want to know what will fail or is the most likely to have the highest stress? under what conditions? Thermal stresses are compensated for in the design.

I agree. All tubular heat exchanger failures I have seen on any brand are in the first bend after the burners. Usually from an overfired condition, moisture or other contaminants in the combustion air etc.